Camptothecin (CPT) and certain of its derivatives are potent antineoplastic agents that are currently the subject of numerous ongoing scientific investigations. Recently, the Untied States Food and Drug Administration approved the first two CPT derivatives (Irinotecan and Topotecan, discussed below) for human use as therapy for various forms of solid neoplasms.
Camptothecin was isolated in 1966 by Wall and Wani from Camptotheca accuminata, a Chinese yew. CPT was subsequently observed to have potent anti-cancer activity and was introduced into human clinical trials in the late 1970's. The closed E-ring lactone form of CPT was noted to be very poorly water soluble (approximately 0.1 microgram of drug dissolving in 1 mL of water). In order for CPT to be administered in human clinical trials it was first formulated with sodium hydroxide. This formulation resulted in hydrolysis of the lactone E-ring of the camptothecin molecule and formed the water soluble carboxylate species. The sodium hydroxide formulation of CPT created a water soluble CPT species that permitted clinicians to administer larger doses of the drug to cancer patients undergoing Phase I and Phase II clinical trials. It was not learned until much later that the carboxylate form of CPT had approximately one-tenth or less of the antitumor potency of the lactone form of CPT. Clinical trials with sodium hydroxide formulated CPT were disappointing due to the frequently observed significant systemic toxicities and the lack of antineoplastic activity, and clinical studies of CPT were halted in the early 1980's.
Further clinical development of CPT derivatives was not pursued until the mid-1980's. At that time it was reported that CPT had a unique mechanism of action involving the inhibition of DNA synthesis and DNA replication by interactions with the ubiquitous cellular enzyme Topoisomerase I (Topo I). This new information about the mechanism of action of CPT derivatives rekindled the interest in developing new Topo I inhibitors as antineoplastic drugs and subsequently several research groups began attempting to develop new CPT derivatives for cancer therapy. In general, it was observed that, like CPT, many of its derivatives were also very poorly soluble in water (less than 1 .mu.g/mL). This low water solubility greatly limited the practical clinical utility of the drug because prohibitively large volumes, of fluid had to be administered to the patient in order to provide an effective dose of the drug. Because of the potent antineoplastic activity and poor water solubility of CPT and many of its derivatives in water, a great deal of research effort was directed at generating new CPT derivatives that were water soluble. This research is discussed below.
As stated earlier, CPT and many of its derivatives (Wall and Wani Camptothecin and Taxol:Discovery to Clinic-Thirteenth Bruce F. Cain Memoral Award Lecture Cancer Research 55:753-760; 1995) are poorly water soluble and are reportedly poorly soluble in a number of pharmaceutically acceptable organic solvents as well. There are numerous reports of newly created water soluble derivatives of CPT (Sawada, S. et al; Kingsbury, W. D. et al., Luzzio et al. Synthesis and Antitumor Activity of Novel Water Soluble Derivatives of Camptothecin as Specific Inhibitors of Topoisomerase I Jour. Med. Chem. 38:395-401; 1995) which have been synthesized in an attempt to overcome some of the significant technical problems in drug administration of poorly water soluble camptothecins to patients with cancer. Several water soluble CPT derivatives have been synthesized in an attempt to address the poor water solubility and difficulties in administration to patients. Well known examples of these water soluble CPT derivatives include: 9-dimethylaminomethyl-10-hydroxy camptothecin (Topotecan), 7-(4-methylpiperazino)methyl!-10,11-ethylenedioxy camptothecin, 7-(4-methylpiperazino)methyl!-10,11-methylenedioxy camptothecin, and 7-ethyl-10-4-(1-piperidino)-1-piperidino!carbonyloxy camptothecin (Irinotecan or CPT-11).
Other substituted CPT derivatives with different solubility and pharmacologic properties have been synthesized as well; examples of these camptothecin derivatives include 9-amino camptothecin and 9-nitro camptothecin which are poorly soluble in both aqueous and nonaqueous media and have been tested in humans. 9-nitro camptothecin is a prodrug of 9-amino camptothecin and spontaneously converts to 9-amino camptothecin in aqueous media and in vivo in mice, dogs and humans (Hinz et al., Pharmacokinetics of the in vivo and in vitro Conversion of 9-Nitro-20(S)-camptothecin to 9-Amino-20 (S)-camptothecin in Humans, Dogs and Mice, Cancer Research 54:3096-3100; 1994).
The pharmacokinetic behavior of 9-nitro camptothecin and 9-amino camptothecin is similar to the water soluble camptothecin derivatives (Topotecan and Irinotecan) in that the plasma half lives are much shorter than the more lipid soluble CPT derivatives. Another major problem with 9-amino camptothecin is that its chemical synthesis using the semisynthetic method is carried out by nitration of CPT, followed by reduction to the amino group, which is a low yield synthesis. In addition, 9-amino camptothecin is light sensitive, heat sensitive and oxygen sensitive which renders the production and stabilization of 9-amino camptothecin difficult. The chemical decomposition reactions of 9-amino camptothecin can result in the formation of compounds that exhibit a large degree of toxicity in nude mice, whereas pure 9-amino camptothecin is significantly less toxic.
9-amino camptothecin is also difficult to administer to patients because it is poorly soluble in both aqueous and organic solvents. 9-nitro camptothecin is easier to produce and is more chemically stable, but with the chemical conversion to 9-amino camptothecin the drug is reportedly susceptible to MDR/MRP mediated drug resistance, which further limits its utility in the unfortunately common setting of drug resistant neoplasms. Based on pharmacokinetic behavior and chemical properties, 9-amino camptothecin is predicted to have reduced tissue penetration and retention relative to more lipid soluble camptothecin derivatives. Further, its poor solubility diminishes the amount of the drug which can cross the blood/brain barrier.
Of this diverse group of substituted CPT derivatives undergoing human clinical development, Irinotecan (CPT-11) has been one of the most extensively studied in Phase I and Phase II clinical trials in human patients with cancer. It is noteworthy that Irinotecan, which is a water soluble prodrug, is biologically inactive and requires activation by a putative carboxylesterase enzyme. The active species of Irinotecan is the depiperidenylated 10-hydroxy-7-ethyl camptothecin (claimed in Miyasaka et al. U.S. Pat. No. 4,473,692 (1984)), which is also known as SN38. SN38 is a toxic lipophilic metabolite which is formed by an in vivo bioactivation of Irinotecan by a putative carboxylesterase enzyme.
SN38 is very poorly soluble in water and has not been directly administered to human patients with cancer. Recently, it has been reported in human patients that SN38 undergoes further metabolism to form a glucuronide species which is an inactive form of the drug with respect to antitumor activity, and also appears to be involved in producing human toxicity (diarrhea, leukopenia) and substantial interpatient variability in drug levels of the free metabolite and its glucuronide.
Irinotecan has been tested in human clinical trials in the United States, Europe and Japan. Nearly 100 patient deaths directly attributable to Irinotecan drug toxicity have been reported in Japan alone. The Miyasaka et al. patents (U.S. Pat. Nos. 4,473,692 and 4,604,463) state that the object of their invention is to "provide 10-substituted camptothecins which are strong in anti-tumor activity and possess good absorbability in living bodies with very low toxicity" and "to provide new camptothecin derivatives which are strong in anti-tumor activity and possess good solubility in water and an extremely low toxicity".
Having multiple drug-related human deaths and serious patient toxicity, is clearly a failure of the Miyasaka et al. inventions to fulfill their stated objects. It is notable that tremendous interpatient variability with regard to drug levels of various forms, drug metabolism, certain pharmacokinetic properties and toxicity has been reported with the use of Irinotecan in human subjects with cancer. Parenteral administration of Irinotecan can achieve micromolar plasma concentrations of Irinotecan that, through metabolism to form SN38, can yield nanomolar concentrations of the active metabolite SN38. It has recently been reported in human subjects that SN38 undergoes further metabolism to form the SN38 glucuronide (Gupta et al. Metabolic Fate of Irinotecan in Humans: Correlation of Glucuronidation with Diarrhea. Cancer Research 54:3723-3725).
This further metabolic conversion of Irinotecan is important, since there is also reportedly large variability in the conversion of Irinotecan to SN38 and large interpatient variability in the metabolism of SN38 to form the inactive (and toxic) SN38 glucuronide in human subjects. (Gupta et al. Metabolic Fate of Irinotecan in Humans: Correlation of Glucuronidation with Diarrhea. Cancer Research 54:3723-3725; 1994 and Ohe, Y. et al., Phase I Study and Pharmacokinetics of CPT-ll with 5-Day Continuous Infusion. JNCI 84(12):972-974, 1992).
Since the amount of Irinotecan and SN38 metabolized is not predictable in individual patients, significant clinical limitations are posed and create the risk of life-threatening drug toxicity, and/or risk of drug inactivity due to five possible mechanisms: (1) conversion of greater amounts of Irinotecan to SN38; (2) inactivation of SN38 by glucuronidation; (3) conversion of SN38 glucuronide to free SN38; (4) lack of antineoplastic activity due to the conversion of lesser amounts of Irinotecan to form SN38; and (5) lack of antineoplastic activity by more rapid and extensive conversion of SN38 to form the glucuronide species. It is important to note that even a doubling of the plasma concentration of the potent Irinotecan metabolite SN38 may result in significant toxicity, because free SN38 exhibits antineoplastic activity at nanomolar concentrations.
Another source of interpatient variability and toxicity is the in vivo de-glucuronidation of SN38 and similar CPT derivatives to produce a free and active species of the drug. Deglucuronidation of a CPT derivative which is susceptible to A-ring glucuronidation, such as SN38, results in an increase in the plasma or local tissue concentration of the free and active form of the drug, and if high enough levels were reached, patient toxicity, and even death may result.
The present invention takes both glucuronide formation and deglucuronidation into account, since these compositions of matter will not undergo extracyclic A-ring or B-ring glucuronidation.